1. Field of the Invention
This invention relates to a powder metallurgy process for manufacturing stainless steel stock. More particularly, this invention relates to a process for producing high purity stainless steel stock from inexpensive oxygen- and carbon-bearing raw materials.
2. Description of the Prior Art
The production of semi-finished metallic stock destined for processing into a final product can be accomplished by conventional metalurgical techniques, e.g., melting, casting and working, thereby producing a wrought product or by powder metallurgy techniques. To obtain a semi-finished wrought product, such as bar, rod, plate, sheet or the like, a melt of a specific chemical analysis is provided. The melt is continuously cast into a slab, billet or bloom or cast into an ingot and then thermo-mechanically worked into the desired semifinished form.
It is well known that wrought alloys have disadvantages and limitations. Some of the more apparent shortcomings include: segregation (which requires processing steps to improve homogeneity, such as working and annealing) work hardening, shape limitations, large sections required and directionality resulting from substantial reductions in area.
Production of semi-finished metallic stock by powder metallurgy techniques offers the following advantages over production of wrought stock:
A smaller intermediate is required, one that is closer to finished size than either a wrought ingot, slab, bloom or billet. For example, a 50 - 100 mil thick sintered compact could yield a 10 mil strip with properties equivalent to a wrought product of similar composition. This eliminates the more conventional large wrought sections that normally require massive equipment to effect reductions in area.
Powder metallurgy stock is also characterized by reduced or minimal segregation. This is due in part to the utilization of a particle mass having a chemical analysis equivalent to the final product. Furthermore, each individual particle can be considered to be micro-ingot having the desired alloy composition. The finished stock might therefore be considered as having, at worst, micro-segregation as opposed to the objectionable macro-segregation characteristic of wrought stock.
Product forms consisting of compositions that work harden easily, such as ferritic stainless steels and cupronickel alloys can now be produced by powder metallurgy. To produce these product forms from alloys in the wrought form requires many processing steps to reach a homogeneous product of the desired dimensions, because with each step that reduces the cross-section an annealing step is required to remove work-hardening and restore ductility. Since powder metallurgy can produce stock closer to the desired final dimensions, the problem of work-hardening is considerably minimized.
Powder metallurgy can also produce stock not easily attainable, if at all, in the wrought form because of control over macro- and micro-structure and density. For example, composite structures and dispersion strengthened materials can now be produced by powder metallurgy.
Although the production of metallic stock by powder metallurgy techniques offers the aforementioned advantages over metallic stock produced by more conventional techniques, powder metallurgy has long been faced with limitations and disadvantages due to certain economic and technical problems. These problems include particle characteristic requirements, difficulties with powder feeding and the relationship of finished stock size and equipment capabilities.
Particle characteristics that effect product and process applications include particle size, shape and distribution, compressibility, flow rate and presence of various interstitial elements, e.g., carbon, nitrogen, sulfur, boron, oxygen and phosphorous. Prior metallurgical processing of the particles, such as annealing, pre-alloying or blending, is a further influencing characteristic. Other significant characteristics include the presence and distribution of various metallic oxides.
To transform a particle mass into a resultant rod, bar, plate or sheet product with sufficient green strength for subsequent handling, a reasonably high compacted density is required. This density is controlled by particle size, shape, distribution, compaction, pressure and compressibility. If the particle mass is a coarse, spherical pre-alloyed powder, the compacting pressure required to produce a compact with sufficient green strength is extremely high or impractical to achieve.
Normally particles have an oxide layer. The presence of this layer interferes with the compacting process. Unless this layer is removed, the finished product will have low ductility and poor mechanical and corrosion properties. When the compact is sintered the oxide layer will be removed by deoxidation. To facilitate deoxidation economically, the compacted particle mass must have some interconnected porosity. Deoxidation can be accelerated by sintering at an elevated temperature for an extended time period and in a dry reducing atmosphere. However, interconnected porosity, during compaction, will considerably reduce the green strength of the compact particularly if a coarse, pre-alloyed powder is used. Even if a suitable compact with interconnected porosity is attained the compact must be subjected to extensive processing in order to obtain a finished product with properties equivalent to a comparable wrought product. Therefore, a compromise is usually reached between green strength and amount of deoxidation with an associated effect on properties.
Compacting pressures can be considerably reduced by the selection of powders that will constitute the particle mass. However, this is not the panacea for curing compacting problems. For example, by using fine, oxide-free, irregular and fullyannealed pre-alloyed stainless steel powders which have a low level of interstitials, a reduced compacting pressure can be employed. However, powders with these properties are expensive, thereby making the use of powder metallurgy stock less attractive from an economic viewpoint. Likewise, using a blend of elemental powders, such as iron, chromium, nickel or manganese, also results in reduced compacting pressures. These powders normally have an oxide film that must be removed. Unless this film is completely removed, the resultant compact will have lower ductility, mechanical properties and corrosion properties than a comparable wrought product.
In direct particle rolling, feeding of blended powders is also a problem because maintaining a uniform rate of feed is difficult with respect to width and thickness. Furthermore, blended powders tend to segregate to a considerable extent during feeding and rolling. For example, in the production of stainless steel stock with a nominal composition of 18 % chromium and 8 % nickel, a blend of iron, chromium, nickel, silicon and manganese powders is employed. These powders have different particle characteristics and during processing into strip or plate tend to segregate toward each other, thereby producing a structure with non-uniform distribution of particles of various compositions.
Direct particle rolling of blended powders into strip usually generates edge and lateral cracking. This phenomena usually increases with increased rolling speed. Furthermore, the powders used in direct particle rolled strip must be completely pre-dried, otherwise the strip will develop cracks and blisters during sintering. It is readily apparent that these defects lower the efficiency of direct particle rolling.
A further limitation in the production of metallic stock via powder metallurgy techniques is that the cross-sectional thickness of the resultant product is limited to relatively thin strips or sheets. As pointed out in U.S. Pat. No. 3,389,993, strip thickness is limited by the diameter of the compacting rolls. To roll compact products of a relatively thick cross-section, very large diameter rolls and supporting equipment is required. For example, to produce a 0.5 inch diameter rod, rolls 250 to 500 inches in diameter would be required. This patent overcomes the restriction on cross-sectional thickness by assembling a plurality of slit green strips, followed by sintering and mechanical working. Although the problem of cross-section is overcome by the teaching of this patent, a second problem of product width is introduced.
The method of this invention offers the typical advantages of powder metallurgy over the conventional production of wrought metallic stock. Furthermore, the method overcomes the above-mentioned limitations normally associated with conventional powder metallurgy techniques.